1. Field of the Invention
The present invention relates to light-emitting elements utilizing organic electroluminescence (EL) (hereinafter, also referred to as organic EL elements).
2. Description of the Related Art
Organic EL elements have been keenly studied and developed (see, Patent Document 1 and Non-Patent Documents 1 and 2). An organic EL element has a basic structure in which a layer including a light-emitting organic compound (hereinafter, also referred to as light-emitting layer) is sandwiched between a pair of electrodes and has features of small thickness and light weight, high speed response to input signals, direct current low voltage driving, and the like; therefore, has been attracting attentions as a next generation flat panel display element. In addition, a display using such a light-emitting element has a feature of high contrast and high image quality, and wide viewing angle. Further, since an organic EL element is a plane light source, it is considered that the light-emitting element is applied as a light source such as a backlight of a liquid crystal display and a lighting device.
The light-emission mechanism of organic EL elements is a carrier-injection system. Namely, by voltage application between electrodes with a light emitting layer interposed between the electrodes, electrons and holes injected from the electrodes are recombined to make the light-emitting substance excited, and light is emitted when the light-emitting substance returns from the excited state to the ground state. There are two types of the excited states: a singlet excited state and a triplet excited state. Further, the statistical generation ratio of the singlet excited state to the triplet excited state in a light-emitting element is considered to be 1:3. Note that, unless indicating it in particular, a singlet excited state (or a triplet excited state) means a singlet excited state (or a triplet excited state) with the lowest energy level among the singlet excited states (or the triplet excited states), in the description.
In general, the ground state of a light-emitting organic compound is a singlet state. Light emission from a singlet excited state is referred to as fluorescence where electron transition occurs between the same spin multiplicities. On the other hand, light emission from a triplet excited state is referred to as phosphorescence where electron transition occurs between different spin multiplicities. Here, in a compound emitting fluorescence (hereinafter referred to as fluorescent compound), in general, phosphorescence is not observed at room temperature; and only fluorescence is observed. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25% based on the ratio of the singlet excited state to the triplet excited state (=1:3).
On the other hand, when a compound emitting phosphorescence (hereinafter referred to as phosphorescent compound) is used, the internal quantum efficiency can be theoretically increased to 100%. In other words, higher emission efficiency can be obtained than that when a fluorescent compound is used. Therefore, the light-emitting element using a phosphorescent compound has been actively developed in recent years in order to achieve a highly efficient light-emitting element.
As the phosphorescent compound, an organometallic complex that has iridium or the like as a central metal has particularly alit acted attentions because of its high phosphorescence quantum yield; for example, an organometallic complex that has iridium as a central metal is disclosed as a phosphorescent material in Patent Document 1.
When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound described above, in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound, the light-emitting layer is often formed such that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound serving as the matrix is called host, and the compound dispersed in the matrix, such as a phosphorescent compound, is called guest.
There are generally given several elementary processes of light emission in a light-emitting element using a phosphorescent compound as a guest like that, and descriptions of the elementary processes are given below.
(1) An electron and a hole are recombined in a guest molecule, and the guest molecule is excited (direct recombination process).
(1-1) When the excited state of the guest molecule is a triplet excited state, the guest molecule emits phosphorescence.
(1-2) When the excited state of the guest molecule is a singlet excited state, the guest molecule in the singlet excited state undergoes intersystem crossing to a triplet excited state, which emits phosphorescence.
In other words, in the direct recombination process in (1), as long as the efficiency of intersystem crossing and the phosphorescence quantum efficiency of the guest molecule are high, a high emission efficiency can be obtained.
(2) An electron and a hole are recombined in a host molecule, and the host molecule is put in an excited state (energy transfer process).
(2-1) When the excited state of the host molecule is a triplet excited state and the energy level in the triplet excited state (T1 level) of the host molecule is higher than that of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, and thus the guest molecule is put in a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. Note that energy transfer to an energy level in a singlet excitation state (S1 level) of the guest molecule is possible in theory; however, in many cases, the S1 level of the guest molecule is higher than the T1 level of the host molecule, and thus such energy transfer to a singlet excitation energy level (S1 level) of the guest molecule is difficult to be a main energy transfer process. Therefore, description thereof is not given here.(2-2) When the excited state of the host molecule is a singlet excited state, in a case where an energy level in a singlet excitation state (S1 level) of the host molecule is higher than the S1 level and T1 level of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, and thus, the guest molecule is put in a singlet excited state or a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. In addition, the guest molecule in the singlet excited state makes intersystem crossing with a triplet excited state, which emits phosphorescence.
In other words, in the energy transfer process in (2), it is important how efficiently both the triplet excitation energy and the singlet excitation energy of the host molecule can move to the guest molecule.
In view of the energy transfer process, before excitation energy is transferred from the host molecule to the guest molecule, the host molecule itself radiates its excitation energy in the form of light or heat to be inactivated, which decreases emission efficiency.
[Energy Transfer Process]
Energy transfer processes between molecules are described below in details.
First, as a mechanism of energy transfer between molecules, the following two mechanisms are proposed. A molecule giving excitation energy is referred to as host molecule, while a molecule taking excitation energy is referred to as guest molecule.
[[Förster Mechanism (Dipole-Dipole Interaction)]]
Förster mechanism (also referred to as Förster resonance energy transfer) does not require direct contact between molecules for energy transfer. Through a resonant phenomenon of dipolar oscillation between a host molecule and a guest molecule, energy is transferred. By the resonant phenomenon of dipolar oscillation, the host molecule gives energy to the guest molecule, and thus the host molecule is put in a ground state and the guest molecule is put in an excited state. The rate constant kh*→g of Förster mechanism is expressed by a formula (1).
                              [                      Formula            ⁢                                                  ⁢                          (              1              )                                ]                ⁢                                                                                                k                                    h              *                        →            g                          =                                            9000              ⁢                              c                4                            ⁢                              K                2                            ⁢              ϕ              ⁢                                                          ⁢              ln              ⁢                                                          ⁢              10                                      128              ⁢                              π                5                            ⁢                              n                4                            ⁢              N              ⁢                                                          ⁢              τ              ⁢                                                          ⁢                              R                6                                              ⁢                      ∫                                                                                                      f                      h                      ′                                        ⁡                                          (                      v                      )                                                        ⁢                                                            ɛ                      g                                        ⁡                                          (                      v                      )                                                                                        v                  4                                            ⁢                              ⅆ                v                                                                        (        1        )            
In the formula 1, v denotes a frequency, f′h(ν) denotes a normalized emission spectrum of a host molecule (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), εg(ν) denotes a molar absorption coefficient of a guest molecule, N denotes Avogadro's number, n denotes a refractive index of a medium, R denotes an intermolecular distance between the host molecule and the guest molecule, τ denotes a measured lifetime of an excited state (fluorescent lifetime or phosphorescent lifetime), c denotes light speed, φ denotes a luminescence quantum efficiency (a fluorescent quantum efficiency in energy transfer from a singlet excited state, and a phosphorescent quantum efficiency in energy transfer from a triplet excited state), and K2 denotes a coefficient (0 to 4) of orientation of a transition dipole moment between the host molecule and the guest molecule. Note that K2=2/3 in random orientation.
[[Dexter Mechanism (Electron Exchange Interaction)]]
In Dexter mechanism (also referred to as Dexter electron transfer), a host molecule and a guest molecule are close to a contact effective range where their orbitals overlap, and the host molecule in an excited state and the guest molecule in a ground state exchange their electrons, which leads to energy transfer. The rate constant kh*→g of Dexter mechanism is expressed by a formula (2).
                              [                      Formula            ⁢                                                  ⁢                          (              2              )                                ]                ⁢                                                                                                k                                    h              *                        →            g                          =                              (                                          2                ⁢                π                            h                        )                    ⁢                      K            2                    ⁢                      exp            ⁡                          (                              -                                                      2                    ⁢                    R                                    L                                            )                                ⁢                      ∫                                                            f                  h                  ′                                ⁡                                  (                  v                  )                                            ⁢                                                ɛ                  g                  ′                                ⁡                                  (                  v                  )                                            ⁢                              ⅆ                v                                                                        (        2        )            
In the formula (2), h denotes a Planck constant, K denotes a constant having an energy dimension, v denotes a frequency, f′h(ν) denotes a normalized emission spectrum of a host molecule (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), ε′g(ν) denotes a normalized absorption spectrum of a guest molecule, L denotes an effective molecular radius, and R denotes an intermolecular distance between the host molecule and the guest molecule.
Here, the efficiency ΦET of energy transfer from the host molecule to the guest molecule is thought to be expressed by a formula (3). In the formula, kr denotes a rate constant of a light-emission process of the host molecule (fluorescence in energy transfer from a singlet excited state of the host molecule, and phosphorescence in energy transfer from a triplet excited state of the host molecule), kr, denotes a rate constant of a non-light-emission process (thermal deactivation or intersystem crossing), and τ denotes a measured lifetime of the excited state of the host molecule.
                              [                      Formula            ⁢                                                  ⁢                          (              3              )                                ]                ⁢                                                                                                Φ          ET                =                                            k                                                h                  *                                →                g                                                                    k                r                            +                              k                n                            +                              k                                                      h                    *                                    →                  g                                                              =                                    k                                                h                  *                                →                g                                                                    (                                  1                  τ                                )                            +                              k                                                      h                    *                                    →                  g                                                                                        (        3        )            
First, according to the formula (3), in order to increase the efficiency ΦET of energy transfer, the rate constant kh*→g of energy transfer should be further increased as compared with another competing rate constant kr+kn (=1/τ). Then, in order to increase the rate constant kh*→g of energy transfer, based on the formulae (1) and (2), in Förster mechanism and Dexter mechanism, the overlapping portion of a light-emission spectrum of a host molecule (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state) and an absorption spectrum of a guest molecule (an energy difference between a triplet excited state and a ground state in the usual case of phosphorescence) is preferably large.
For example, by materials selected such that an energy difference between the triplet excited state and the ground state of the host molecule overlaps with an energy difference between the triplet excited state and the ground state of the guest molecule, energy is efficiently transferred from the host to the guest.
However, such energy transfer occurs similarly in transfer from a guest molecule in a triplet excited state to a host molecule in a ground state. By materials selected such that an energy difference between the triplet excited state and the ground state of the host molecule is equal or close to an energy difference between the triplet excited state and the ground state of the guest molecule, energy is easily transferred from the guest molecule in the triplet excited state to the host molecule in the triplet excited state. As a result, emission efficiency is decreased unfortunately.
Against such a problem, for example, as described in Non-Patent Document 1, there is proposed a method in which the energy difference between the triplet excited state and the ground state of a host molecule is made larger than the energy difference between the triplet excited state and the ground state of a guest molecule.
In Non-Patent Document 1, the energy difference between the triplet excited state and the ground state of a host molecule is larger than the energy difference between the triplet excited state and the ground state of a guest molecule by 0.3 eV (at present, it is converted into 0.15 eV), and thereby transition does not occur from the triplet excited state of the guest molecule to the triplet excited state of the host molecule.
In other words, the energy difference between the triplet excited state and the ground state of a host molecule is larger than the energy difference between the triplet excited state and the ground state of a guest molecule by 0.15 eV and thereby, transition from the triplet excited state of the guest molecule to the triplet excited state of the host molecule can be inhibited well.